The electrolytic tinning of steel strip to produce a material referred to as "tinplate" is well known in the art. The process is described in detail in the book "The Technology of Tinplate" by Hoare et al., published by St. Martin's Press, New York, 1965.
Strip steel is electroplated with tin in large continuous plating machines at steel mills throughout the world. In these machines, a large coil of steel sheet unwinds at one end of the machine and proceeds through cleaning and acid pickling stations, followed by multiple tin electroplating stations to produce a tin deposit over the steel surface. The tin coating, as plated, exhibits a characteristic smooth matte surface.
The next major section of the line is variously known as the "flow-melting", "flow-brightening", or "reflow" section. The reflow operation is used to transform the matte deposit to the bright reflective finish typical of tinplate and to produce a thin iron-tin compound layer at the interface between the tin coating and the steel base, thereby improving corrosion resistance. The operation includes the steps of raising the temperature of the tin coating to above the melting point of tin, followed by immediate quenching to impart and achieve the desired properties of the deposit.
In the reflow operation, after the matte tinplate is rinsed, the steel sheet proceeds through a fluxing station. The term "flux" in this context refers to a substance that aids, induces, or otherwise actively participates in fusing or flowing. The application of flux is followed by the reflow station itself which raises the temperature of the steel to slightly above the melting point of tin. The steel is then quickly quenched in water, resulting in a tin surface that has a bright finish. After reflow, the steel proceeds through other stations for treatments such as passivation, oiling, and rewinding or cutting into sections at the exit-end of the machine.
A uniform, bright finish is achieved without blemishes or discontinuities if all of the above steps are carried out to perfection. A flux treatment prior to reflow is important to prevent formation of tin oxides or hydroxides in the reflow process which would otherwise cause defects in the tin finish during reflow. The most serious defect that is generally encountered is referred to as "woodgrain". Woodgrain is a defect in reflowed tinplate in which the surface takes on the non-uniform appearance of polished wood. Other common defects are a blue haze effect and white staining.
Prior art fluxes are all acidic in nature and consist of an aqueous solution of an acid such as hydrochloric acid or phenolsulfonic acid or of an acid salt such as ammonium chloride or zinc chloride. This type of flux treatment was developed in 1957 and was described in "A History of Tinplate" by Dr. Anthony Smith published in Tin International, March 1977 to June 1978.
Belousova et al. in Stal (USSR), May 1991, describe the use of phenolsulfonic acid in a fluxing solution. Tinplate was obtained from a phenolsulfonic acid-based solution that contained naphthoxol-7S as an additive to achieve fine grained tin deposits. However, phenolsulfonic acids are pollutants which cannot be discharged into the environment.
Accordingly, there remains a need for improved fluxes which can achieve the desired properties of the reflowed tinplate while at the same time not utilizing chemicals which present disposal and pollution problems.